(1) Field of the invention
The present invention generally relates to a battery feed circuit, and more particularly to a battery feed circuit for supplying a feed current to a telephone set via a subscriber line.
(2) Description of related art
A battery feed circuit supplies a feed current to a telephone set via a 2-wire telecommunication line (a subscriber line). In a conventional battery feed circuit, a passive circuit having coils has been used. Further, due to development of electronic circuits, a battery feed circuit using an electronic circuit has been proposed recently.
A conventional battery feed circuit using a electronic circuit is shown in FIG. 1. This conventional battery feed circuit is disclosed in U.S. Pat. No. 4.476,350.
Referring to FIG. 1, a feed current is supplied to a telephone set 1 via a 2-wire telecommunication line formed of a TIP line and a RING line. The TIP line is connected to a hi-directional constant-current source U5 and the RING line is connected to a bi-directional constant-current source U6. A structure of each of the bi-directional constant-current sources U5 and U6 is disclosed, for example, in "IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 4 August 1981, High Voltage IC for a Transformerless Trunk and Subscriber line Interface". The hi-directional constant-current sources U5 and U6 supply the current respectively to the TIP line and the RING line. The bi-directional constant-current sources U5 and U6 carry out complementary operations, so that, for example, a current output from the hi-directional constant-current source U5 passes through the TIP line, the telephone set. 1 and the RING line and is absorbed into the bi-directional constant-current source U6. A voltage VTR across the TIP line and the RING line is supplied to a differential amplifier U1. The differential amplifier U1 detects the voltage VTR between the TIP line and the RING line. Circuit constants in the differential amplifier U1 are set to predetermined values so that the output of the differential amplifier U1 has a value of EQU VTR.times..alpha.(.alpha.&lt;1).
The output of the differential amplifier U1 is supplied to an inverting amplifier U2 formed of an operational amplifier, a resistor R1 and a feedback resistor R2. A point between the resistors R1 and R2 is connected with a constant-current source I1. The current output of the constant-current source I1 is set to a value so that the TIP line and the RING line are biased at constant voltages respectively against a ground and a power source VBB when no feed current is supplied to the telephone set in a state where the telephone set is on the hook (on-hook state). A current passing through the resistor R1 is equal to a value of VTR.times..alpha./R1 (hereinafter R1 indicates both the resistor and a resistance thereof). A current i1 output from the constant-current source I1 is subtracted from the above current (VTR.times..alpha./R1), and a current obtained by the subtraction passes through tile feed back resistor R2 of the inverting amplifier U2, so that the inverting amplifier U2 outputs a voltage Vo. In this case, the following formula stands. EQU [(VTR.times..alpha./R1)-i1].times.R2=-Vo
Thus, the output voltage Vo of the inverting amplifier U2 is denoted by the following formula (1). EQU Vo=-[(VTR.times..alpha./R1)-i1].times.R2 (1)
The output voltage Vo is supplied to a base of a transistor Q31 in a differential amplifier U8 via a low pass filter LP. The differential amplifier U8 outputs a first current and a second current respectively via transistors Q31 and Q32. The first and second currents are respectively supplied to amplifiers U3 and U4. When the voltage Vo supplied to the differential amplifier U8 is equal to zero (vo=0), the first and second currents are equal to each other and each of the currents is I2/2. When the voltage Vo is not equal to zero (Vo.noteq.0), the first current is [(I2/2)-(Vo/2R3)]and the second current is [(I2/2)+(Vo/2R4)]. The amplifiers U3 and U4 convert the input currents into voltages, and voltages output from the amplifiers U3 and U4 are respectively supplied to the bi-directional constant-current sources U5 and U6. A differential voltage between the outputs of the amplifiers U3 and U4 is in proportion to the voltage Vo and a phase of the differential voltage differs from that of the voltage Vo by 180.degree.. Thus, the bi-directional constant-current sources U5 and U6 output currents respectively in proportion to (-Vo/R3) and (+Vo/R4). That is, the hi-directional constant-current sources U5 and U6 carry out complementary operations so that the feed current is supplied to the telephone set 1 via the TIP line and the RING line.
In the circuit shown in FIG. 1, the current il from the constant-current source i1 corresponds to the output voltage VBB of the power supply. From the above formula (1), it is understood that the output voltage Vo of the inverting amplifier U2 corresponds to (VBB VTR). VTR). The outputs of both the amplifiers U3 and U4 are not symmetrical about the value of VBB/2, but feedback operations of a common mode signal detecting circuit U7 are strongly applied to the hi-directional constant-current sources U5 and U6 so that the output of the common mode signal detecting circuit U7 cancels the current I2. As a result, the voltages of the TIP line and the RING line are balanced.
In the conventional battery feed circuit described above, the feed current supplied to the telephone set 1 is controlled based on the differential voltage VTR across the TIP line and the RING line. That is, the differential voltage VTR is detected and converted into a current, and then the feed current supplied to the telephone set 1 is controlled based on the voltage Vo corresponding to the current obtained by subtracting the current i1 from the above current obtained based on the differential voltage VTR. As the feed current supplied to the telephone set 1 is controlled based on the differential voltage VTR across the TIP line and the RING line, even if the output voltage VBB (e.g. -48V) of the power supply varies, the feed current supplied to the telephone set 1 via the TIP line and the RING line is maintained at a constant value. However, when the output voltage VBB of the power supply varies, a voltage VRB across the RING line and a power line (VBB) of the power supply, and a voltage VTG across the TIP line and the ground line (0v) vary. Thus, when the output voltage VBB of the power supply decreases, a dynamic range on each of the TIP line and the RING line also decreases. As a result, in a case where line resistances of the TIP line and the RING line are large (the telephone set 1 is located away from the battery feed circuit), there is a possibility that transistors in the battery feed circuit are saturated and voice signals received and transmitted to and from the telephone set 1 via the TIP line and the RING line are distorted.